Although the role of microtubules within cells is poorly understood, it is known that they are important for the maintenance of cell shape and for the movements of secretory vesicles and other membrane-bounded organelles within cells. Morphological studies have shown direct contacts between microtubules and organelle membranes via filamentous structures in a large number of cells. Biochemical and morphological studies of microtubules which have been assembled in vitro from tissue homogenates have revealed that these filamentous structures are attached to the walls of microtubules and that they are comprised of one or two molecular weight proteins. Since they co-assemble with microtubules, they are referred to as microtubule-associated proteins, or MAPs. The overall objectives of this project are to study the mechanisms by which microtubule proteins and actin interact with one another as well as with cellular membranes and membrane-bounded cytoplasmic organelles. The studies that were carried out in the current year have shown that microtubules can associate witth vesicles in vitro and that certain proteins are associated with microtubules are responsible for the interactions. Other studies have shown that microtubule-associated proteins can associate with actin filaments to form specific filamentous structure in vitro.